You're not trying to power the motorshield from the Arduino 5V rail perchance? Motors need a separate supply (and given that that shield uses the L293D it should be about 2V higher than you want to reach the motors).
You're not trying to power the motorshield from the Arduino 5V rail perchance? Motors need a separate supply (and given that that shield uses the L293D it should be about 2V higher than you want to reach the motors).